When screening paper pulps containing a variety of impurities in a screen described e.g. in U.S. Pat. No. 3,363,759, the reject contains also particles which can be separated from it by employing centrifugal force. The particles separated in this way, the specific gravity of which is lower than that of the average particles, i.e. the light reject, contains e.g. various plastics. The remaining reject contains e.g. fibre bundles and pieces of knots. It is an object of the present invention to provide a screen which in a simple manner separates light impurities from the reject flow.